The closed-loop pre-coding technology has been introduced into the Long Term Evolution (LTE) Release 8 (Rel-8) system to improve the spectrum efficiency. Firstly the same set of pre-coding matrixes, referred to as a codebook, for closed-loop pre-coding is required to be stored in both a base station and a terminal. The terminal estimates channel information according to a cell common pilot, and then selects a pre-coding matrix from the codebook according to some criterion. The selection criterion can be maximizing mutual information, or maximizing output Signal to interference plus Noise Ratio (SINR), etc. The terminal feeds an index of the selected pre-coding matrix in the codebook back to the base station over an uplink channel, where the index is referred to as a Pre-coding Matrix Indicator (0). The base station can determine the pre-coding matrix to be used for the terminal, according to the received index. The pre-coding matrix reported by the terminal can be regarded as quantified CSI.
A transmission parameter is selected according to the feedback from the terminal in the closed-loop pre-coding technology. In a scenario where the terminal is moving at a high speed, the information fed back by the terminal may be outdated so that the transmission parameter mismatches a real channel condition, thus degrading the performance of the system. In order to cope with the scenario where the terminal is moving at a high speed, the open-loop Multiple Input Multiple Output (MIMO) transmission scheme has been introduced into the LTE Rel-8. In the open-loop MIMO transmission scheme, data is transmitted by using pre-coding matrixes in a predefined set alternately. The terminal only needs to feed back Channel Quality Indicator (CQI) information and Rank Indicator (RI) information without feeding back any PMI. The terminal calculates the CQI and the RI on the assumption that data is transmitted over resources by using the predefined pre-coding matrixes alternately.
As the antenna technology is advancing, active antennas whose array elements can be controlled separately become mature. FIG. 1 is a schematic structural diagram of dual polarized antennas arranged in the horizontal and vertical dimensions, and FIG. 2 is a schematic structural diagram of a linear antenna array arranged in the horizontal and vertical dimensions. As illustrated by FIGS. 1 and 2, antenna elements in an active antenna array can be arranged in a two dimensional plane to greatly reduce the size of the antenna array. A large-scale antenna array is generally embodied as two dimensional planar antenna arrays. Moreover, by using this kind of antenna array, a beam can be adjusted dynamically in the horizontal and vertical directions for more flexible beam-forming. Beam-forming in the horizontal and vertical directions using a two dimensional planar array is also referred to as Full-Dimension MIMO (FD-MIMO). FD-MIMO depends upon the CSI reported by the terminal, but the CSI reported by the terminal moving at a high speed may be unreliable.
As such, the predefined set of pre-coding matrixes is polled in the open-loop MIMO solution defined in the LTE Rel-8, and actually the space is traversed as a result of polling. However a drawback thereof lies in that a beam in the space tends to become narrower as the number of antennas increases, and thus to traverse the space, a large number of pre-coding matrixes need to be traversed, thereby significantly limiting implementation of the system, and failing to guarantee the performance thereof.